Elegant and groundbreaking, this book advances a general theory of security relationships in trying to answer a puzzle: why do states sometimes choose to provide for their own security and at other times join alliances? This choice has loomed particularly large for the United States this century as it periodically agonizes about whether to go it alone or entangle itself with others. The author argues that a nation's security relations -- whether unilateral, allied, or imperial -- depend on the costs and benefits of each option. Alliances are more attractive when they provide gains based on the economies of scale, but cooperation also erodes autonomy and opens the possibility of exploitation. In short, any decision always involves a tradeoff. Lake uses this framework to illuminate America's historic choices after the two world wars and the Cold War. His controversial conclusion: the United States chose engagement after 1945 and not in 1919 because the gains from cooperation were greater the second time around. Scholars will debate whether America's rejection of the League of Nations was really an "efficient" choice, and many will ask how Lake can explain America's security commitment to Europe after the Cold War. Nonetheless, a useful and insightful framework to guide debates over American foreign policy.